(a) Field of the Invention
This invention relates to a method of producing a gel for use in liquid chromatography, and more particularly to a method of producing hard polyvinyl alcohol (hereinafter referred to as PVA) gel characterized by copolymerizing a carboxylic acid vinyl ester monomer with a cross-linking agent having a particular structure and then saponifying the copolymer.
(b) Description of the Prior Art
In recent years, liquid chromatography is not only used as an analytical means but also it is developing its application as an industrial separation means in many fields such as food industry, pharmaceutical industry, synthesis and purification of intermediate products in chemical industry, inorganic chemical industry, textile industry, etc. As gels for such use in liquid chromatography, cross-linked dextran polymer gel, cross-linked acrylamide polymer gel, agar, agarose gel, are known.
However, these aqueous gels become very soft with an increase of water content and become weak in mechanical strength. Therefore, they are susceptible to fracture upon mechanical operations such as column packing, stirring, etc. They also have a defect that they are not strong enough against operations under pressure.
Among gels which do not suffer from weak gel strength peculiar to such conventional aqueous gels, PVA gels structurally strengthened in cross-linking power are known as aqueous gels for use in liquid chromatography. For example, W, Heitz et al report on a PVA gel obtained by saponifying polyvinyl acetate cross-linked with butane diol divinyl ether (Makromolekulare Chemie, 98, 42 (1966)). Furthermore, in Japanese Patent Kokai (Laid open) No. 138077/77, a PVA gel is produced by alkali-saponifying a copolymer of vinyl acetate and diethylene glycol dimethacrylate, etc. and further cross-linking the saponified copolymer with epichlorohydrin.
However, as is well known, cross-linking agents such as diethylene glycol dimethacrylate, butane diol divinyl ether, etc. are also hydrolyzed during saponification of polyvinyl acetate, and therefore it is very difficult to maintain sufficient cross-linkage of the initial stage. In consideration of this fact, Japanese Patent Kokai No. 138077/77 adopts the method of re-cross-linking with epichlorohydrin after the saponification of polyvinyl acetate.
On the other hand, Japanese Patent Publication No. 3482/83 and Japanese Patent Kokai No. 30945/82 propose a method of Producing a PVA gel which requires only cross-linking upon copolymerization but requires no additional cross-linking after saponification, wherein a cross-linking agent having a triazine ring structure is copolymerized. However, such a cross-linking agent has a defect of poor copolymerizability with vinyl carboxylate or slow polymerization speed, and there was a limit in obtaining an efficient cross-linked structure, so that it was impossible to obtain a hard PVA gel having a sufficient mechanical strength.